Field of the Invention
The present invention relates to a machine for applying threaded caps to containers.
Description of Prior Art
A common technique for closing containers such as bottles and the like envisages forming an external thread on the neck of the containers and applying caps with an internal thread to the containers.
The machines usually used for applying threaded caps to containers comprise a carousel structure that picks up the containers to be closed and makes them advance along a circular path. The carousel structure carries a plurality of capping heads each of which is equipped with a respective cap-gripping member. The cap-gripping member is driven with a rotational movement about a respective longitudinal axis and a translational movement. The rotational movement and the translational movement of the gripping member are mutually synchronized so as to apply a screwing movement to the caps.
In more traditional solutions, the screwing movement of the caps onto the containers stops when a preset torque is reached. The screwing heads limit the maximum torque transmitted to the cap-applying member by means of clutches, for example of the magnetic type, or thanks to a current-limited electric motor.
These solutions are not entirely satisfactory because the scewing of the caps based on the maximum torque can produce defective closures.
To overcome these drawbacks, systems have already been proposed for applying threaded caps that apply a screwing stroke of preset angular amplitude starting from a condition of incipient coupling between the thread of the cap and the thread of the container.
EP-A1-1491490 describes a screwing head equipped with a torque sensor that detects the screwing torque applied to the cap during screwing onto the container. The measured torque rapidly increases in the position wherein the threads of the cap and of the container come into mutual contact, in this way signalling the position of incipient coupling between the thread of the cap and the thread of the container. The cap is rotated by a predetermined angle with reference to the position of incipient coupling.
US2011/0083405-A1 describes a screwing head that detects the condition of incipient coupling between the cap and the container by measuring the current absorbed by the motor which drives the screwing head.
In these known solutions the condition of incipient coupling between the threads is detected after the cap is applied onto the container. A part of the working stroke of the screwing head is used for the recognition of the position of incipient coupling between the cap and the container rather than for the actual screwing of the cap. Consequently, in these known solutions a reduced angular stroke is available for the actual screwing of the cap.